Knuckle boom cranes have long since been used and are advantageous in offshore industries, in part, because of their relatively compact foot print and their ability to provide a relatively low boom tip height. For example, a knuckle boom crane may have a foot print of approximately half of the diameter of a comparable capacity lattice boom crane and the articulating inner and outer boom allows for lowering the boom tip to reduce the pendulum length between the boom tip and the picking hook. The smaller footprint offers advantages where the available area for equipment, material, workers, and working area is relatively small such as on a ship or oil platform. The lower boom tip and shorter pendulum length helps to reduce the swaying motion of a suspended load that may be induced by waves in the ocean, sea, or other waterway. The reduced swaying of the material can provide for more efficient handling of the material and can make for a safer working environment. However, the versatility of knuckle boom cranes can cause them to sacrifice lifting capacity.
Demands for higher capacity cranes continue to increase and demands for cranes that can access deeper waters also continue to increase. Where 100 to 250 metric ton cranes were previously sufficient, industry has demanded more capacity and 400 metric ton cranes have become commonplace. Where depths of 500 meters were previously sufficient, industry has demanded access to deeper waters and 1000 meter depths have become common place. Demands continue to increase and the industry is now requesting 600 metric ton, 700 metric ton and even 800 metric ton cranes. Moreover, not only does the industry want the higher lift capacity, the industry also wants to be able to access ocean depths of 3500 meters; more than 2 miles below the surface.
Solutions to achieve current capacities and payout lengths have involved increasing the cable diameter and length of cable. Each of these changes causes the cable spools (hoists) and associated wire ropes to increase in diameter and weight. The increase in cable/rope diameter and weight has led to relocating the spool (hoist) from the base of the crane to a location below the deck of ships, for example. However, the current demands have exhausted the capacity of this solution. That is, the cable spools (wire ropes) have reached a size and a weight that suppliers of currently available cabling (wire ropes) simply do not have the space in their facilities to produce such large spools of cable (wire ropes). For example, a spool for a cable for a 800 metric ton knuckle boom crane that can reach depths of 3500 meters would have wire rope with a diameter of 165 mm and a weight of approximately 460 metric tons. This solution has run its course and the industry is in need of alternative solutions.